(1) Field of the Invention
The present invention relates to a structural adhesive for use in bonding metals, paper, plastics and inorganic materials. More particularly, it relates to a structural adhesive having a good bonding property even to a surface on which a rust preventive oil or processing oil remains, the adhesive also having a high shearing force and a bonding force, and having an excellent flexural strength even after heating and curing. The adhesive also has an excellent rust preventive effect after bonding.
(2) Description of the Related Art
As known structural adhesives, there can be mentioned epoxy resin type adhesives, phenolic type adhesives, polyurethane type adhesives, structural acrylic adhesives and anaerobic structural adhesives. As the oil-absorbing adhesive capable of bonding to an oil-adhering metal surface, chloroprene type mastic adhesives having a non-volatile component content of at least 67% and vinyl chloride type mastic adhesives having a non-volatile component content of at least 92% are used. The adhesive power of these adhesives is low on an oily surface or, even if the adhesive bonds to the oily surface, the bonding strength is not as high as the bonding strength of the structural adhesive. Furthermore, polyurethane type adhesives, structural acrylic adhesives and anaerobic structural adhesives have a problem in that, when these adhesives are fixed to the surfaces of adherents or the bonded adherents are heated after fixation while being maintained in the vertical state, the adherents are readily moved out of position by a slight shock or vibration. Accordingly, it is necessary to secure the adherents by spot welding or clipping until a sufficient bonding strength is manifested.
Normal temperature curing type adhesives have been reported. For example, Japanese Unexamined Patent Publication No. 50-158624 discloses an adhesive composition comprising a liquid formed by incorporating a liquid chloroprene polymer with an epoxy resin and a liquid comprising a polyamide resin, a xylene resin being incorporated in at least one of the two liquids. However, this adhesive composition exhibits a poor bonding strength at elevated temperatures.
Polyurethane type adhesives are effective for bonding mortar, slate and plywood, but, when used for bonding metal adherents, the adhesives per se readily bubble due to the generation of carbon dioxide gas during the reaction of NCO groups and, therefore, the bonding strength is poor. In cyanoacrylate type adhesives, there is no substantial reduction of the bonding power at temperatures ranging from room temperature to about 90.degree. C., but if the temperature exceeds this range, the bonding power is abruptly reduced and the resistance against peeling by bending is poor. Moreover, since the manufacturing cost is high, these adhesives have little general-purpose utility.
A structural adhesive having a low electric resistance has not been developed. Accordingly, when metal plates are bonded by using conventional structural adhesives and electrodeposition coating is carried out on the bonded metal plates, electrodeposition is possible only on the metal plates because they are electrically conductive, but since most of the structural adhesives have an electric resistance higher than 10.sup.8 .OMEGA.-cm, no electric current flows through the adhesives or, if any electric current flows, the quantity is very small. Therefore, electrodeposition coating cannot be effected in the vicinity of the adhesives or in the bonding interfaces. Accordingly, the generation and growth of rust are readily caused at bonding points of the metals and in the vicinities thereof.